Iterative approach to Stimulated Raman Scattering (SRS) error estimation

ABSTRACT

Iterative approach to Stimulated Raman Scattering (SRS) error estimation in an optical fiber network comprised of determining a multi-channel SRS error value for each pairing of all possible pairings of all the channels by inputting measured channel power levels into a two-channel equation and further calculating the SRS error in a single fiber span for all channels by extending the two-channel equation in an iterative form for calculating the SRS error in a single fiber span for all channels. An embodiment of the invention further comprising determining for each channel over multiple fiber spans the amount of total SRS error value at every span based on the calculated SRS error value of its previous span to determine the SRS error accumulated over a multi-span network.

FIELD OF THE INVENTION

[0001] The present invention relates to maintenance of an optical fibernetwork and more specifically to Stimulated Raman Scattering (SRS) errorestimation.

BACKGROUND OF THE INVENTION

[0002] Today's optical fiber networks carry many channels along theiroptical fibers. A significant challenge in maintaining these networks isthe problem of power level estimation within these channels at everypoint in the network or in other words optical performance monitoring. Asimple tool for optical performance monitoring and channelidentification in DWDM (Dense Wave Division Multiplexing) systems is toadd small signal sinusoidal dithers (pilot tones) to optical carriers.Consequently, each optical carrier has a unique sinusoidal dither whoseamplitude is proportional to the average power of its carrier. Thesepilot tones are superimposed to the average power of the optical channeland can be separated and analysed easily. The presence of a specificdither at a particular point in the network therefore indicates thepresence of its corresponding wavelength and its amplitude will show theaverage optical power.

[0003] This is true when each dither travels solely with its opticalcarrier. However, an effect known as Stimulated Raman Scattering (SRS)precipitates an inter-channel energy transfer that interferes with theability to accurately estimate power levels through pilot tones. Thisinter-channel energy transfer occurs from smaller wavelengths to largerwavelengths causing larger wavelength power levels to increase. SRS notonly causes an interaction between the average power of each channel butalso brings about a transfer of dithers between different channels.Therefore, some of the dither of each channel is transferred to otherchannels and hence its amplitude will not be proportional to the powerof its carrier any more. This causes inaccuracy in power levelestimation using pilot tones.

[0004] SRS energy transfer, or SRS error, largely depends upon thenumber of channels in the network. The more channels present the moreenergy transfer occurring. In a multi-span optical networks, in additionto the number of channels, the number of spans contributessignificantly. In such a system SRS error is accumulated over all thespans causing an even more severe degradation in power level estimation.

[0005] In a multi-span system estimation of the SRS error needs twosteps to be taken. In the first step an estimate of the SRS error in onespan should be obtained. Using the results of the first step, in thesecond step the accumulation of the SRS error over all spans should beconsidered. Once the total error is estimated a compensation algorithmcan be developed to correct for the inaccuracy in pilot tone powerestimation due to SRS.

[0006] For the foregoing reasons, a need exists for an improved methodof SRS error estimation in a single span of an optical fiber networkthat can be applied to a multi-span approach of SRS error estimationwithin a multi-span network.

SUMMARY OF THE INVENTION

[0007] The present invention is directed to an iterative method forStimulated Raman Scattering (SRS) error estimation in an optical fibernetwork, the network characterized in that it comprises theinfrastructure required to measure the power levels of all opticalchannels using a pilot tone monitoring technique, the method comprisingthe steps of determining the multi-channel SRS error value for eachpairing of all possible pairings of all the channels by inputtingmeasured values of channel power levels into a two-channel equationsubstantially equal to${P_{s}(z)} = {P_{s0}{^{{- \alpha}\quad L}\left\lbrack {\left( {1 + \frac{P_{p0}g}{\alpha}} \right) + {\frac{P_{p0}g}{\alpha}m\quad {\cos \left( {\omega \quad t} \right)}}} \right\rbrack}}$

[0008] and calculating the SRS error in a single fiber span for allchannels by extending the two-channel equation in an iterative formsubstantially equal to for k=1:2 for i=1:Nch for j=1:NchX(i)=X(i)+0.5*G*(j−i)*SpanP(j)*SpanTxP0; end SpanP(i)=SpanP(i)−X(i); endend

[0009] to estimate the SRS error in a single span.

[0010] where Nch=Number of channels

[0011] SpanTxP0=Launch power into the fiber (equal power per wavelengthis

[0012] assumed) SpanP(j)=output power at channel j

[0013] X(i)=SRS error in channel i

[0014] G=g/α

[0015] In an aspect of the invention, the method comprises determiningfor each channel over multiple fiber spans the amount of total SRS errorvalue at every span based on the calculated SRS error value of itsprevious span so as to determine the SRS error accumulated over amulti-span network, the method comprising an iterative form of thetwo-channel equation substantially equal to$R_{k} = {{\frac{R_{k - 1}P_{0}}{\left( {R_{k - 1} + E_{k - 1}} \right) + x_{1}^{\prime} + {x_{2}^{\prime}A_{2}\beta_{2}}}\quad E_{k}} = \frac{\left( {x_{1}^{\prime} + {x_{2}^{\prime}A_{2}\beta_{2}} + E_{k - 1}} \right)P_{0}}{\left( {R_{k - 1} + E_{k - 1}} \right) + x_{1}^{\prime} + {x_{2}^{\prime}A_{2}\beta_{2}}}}$

[0016] The advantage of using this iterative approach is to account forthe power depletion in a practical manner. This iterative approach isvery suitable for software implementation and provides adequate accuracyin power readings up to 40 optical channels.

[0017] Other aspects and features of the present invention will becomeapparent to those ordinarily skilled in the art upon review of thefollowing description of specific embodiments of the invention inconjunction with the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] These and other features, aspects, and advantages of the presentinvention will become better understood with regard to the followingdescription, appended claims, and accompanying drawings where:

[0019]FIG. 1 is a flow chart showing the iterative method for StimulatedRaman Scattering (SRS) error estimation in an optical fiber network;

[0020]FIG. 2 is a flow chart showing the iterative method for StimulatedRaman Scattering (SRS) error estimation in an optical fiber networkfurther including a multi-span solution;

[0021]FIG. 3 is a flow chart showing the system for Stimulated RamanScattering (SRS) error estimation in an optical fiber network;

[0022]FIG. 4 is a flow chart showing the system for Stimulated RamanScattering (SRS) error estimation in an optical fiber network furtherincluding a multi-span solution;

[0023]FIG. 5 is a graph displays the results of this approach for 40channels with 6.0o dBm average power per wavelength in an 80km NDSFfiber; and

[0024]FIG. 6 is a graph illustrating the SRS error in pilot tone powerestimation for 40 channels 6-span NDSF system with 6.0 dBm launch powerper wavelength.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENT

[0025] As shown in FIG. 1, the present invention is directed to aniterative method for Stimulated Raman Scattering (SRS) error estimationin an optical fiber network, the network characterized in that itcomprises the infrastructure required to measure the power levels of alloptical channels using a pilot tone monitoring technique, the methodcomprising the steps of determining the multi-channel SRS error valuefor each pairing of all possible pairings of all the channels byinputting measured values of channel power levels into a two-channelequation substantially equal to${P_{s}(z)} = {P_{s0}{^{{- \alpha}\quad L}\left\lbrack {\left( {1 + \frac{P_{p0}g}{\alpha}} \right) + {\frac{P_{p0}g}{\alpha}m\quad {\cos \left( {\omega \quad t} \right)}}} \right\rbrack}}$

[0026] 12 and calculating the SRS error in a single fiber span for allchannels by extending the two-channel equation in an iterative formsubstantially equal to for k=1:2 for i=1:Nch for j=1:NchX(i)=X(i)+0.5*G*(j−i)*SpanP(j)*SpanTxP0; end SpanP(i)=SpanP(i)−X(i); endend

[0027] to estimate the SRS error in a single span.

[0028] where Nch= Number of channels

[0029] SpanTxP0= Launch power into the fiber (equal power per wavelengthis

[0030] assumed) SpanP(j)=output power at channel j

[0031] X(i)=SRS error in channel i

[0032] G=g/α 14

[0033] As shown in FIG. 2, in an embodiment of the invention, the methodcomprises determining for each channel over multiple fiber spans theamount of total SRS error value at every span based on the calculatedSRS error value of its previous span so as to determine the SRS erroraccumulated over a multi-span network, the method comprising aniterative form of the two-channel equation substantially equal to$\begin{matrix}{R_{k} = {{\frac{R_{k - 1}P_{0}}{\left( {R_{k - 1} + E_{k - 1}} \right) + x_{1}^{\prime} + {x_{2}^{\prime}A_{2}\beta_{2}}}\quad E_{k}} = \frac{\left( {x_{1}^{\prime} + {x_{2}^{\prime}A_{2}\beta_{2}} + E_{k - 1}} \right)P_{0}}{\left( {R_{k - 1} + E_{k - 1}} \right) + x_{1}^{\prime} + {x_{2}^{\prime}A_{2}\beta_{2}}}}} & 16\end{matrix}$

[0034] By applying an iterative approach the two wavelength solution canbe extended for an arbitrary number of channels, permitting the effectof power depletion to be included in the results.

[0035] Two factors should be taken into account when determining SRSerror over a multi-span network. First, how the SRS error is manifestedin one span and second, how the SRS error is accumulated over multiplespans. The contribution of both of these factors can be calculated by anappropriate iterative approach as described below.

[0036] For this purpose, an iterative approach is first applied to findthe amount of SRS error in a single span. Next, an iterative algorithmis designed to calculate the amount of SRS error accumulated overmultiple spans by considering the role of the dual and boosteramplifiers in the system.

[0037] With respect to this second iterative approach, it is assumedthat every dual amplifier has a flat gain and every booster amplifierhas a flat power distribution. The total amount of SRS error at everyspan is then calculated based on the error of its previous span.Finally, the total SRS error is used to compensate for the inaccuracy inpower readings.

[0038] When a set of optical channels travels through a fiber,Stimulated Raman Scattering (SRS) causes an energy transfer from shorterto longer wavelengths. This energy transfer causes an inaccuracy in theestimated power readings. In a multi-span system the SRS error in eachspan is accumulated at the end of the system. In this report aniterative approach is proposed to calculate the amount of SRS error forone span. Then a recursive equation will be developed to obtain theaccumulation of the SRS error in a multi-span system.

[0039] Step 1: SRS error estimation in one span:

[0040] In this step we solve the SRS differential equation for twochannels only. Then we extend the results for a multi-channel system.The SRS describing equation for two wavelengths in a single fiber isgiven by: $\begin{matrix}\left\{ \begin{matrix}{{\frac{P_{p}}{z} + {\alpha \quad P_{p}} - {{gP}_{p}P_{s}}} = 0} \\{{\frac{P_{s}}{z} + {\alpha \quad P_{s}} + {{gP}_{p}P_{s}}} = 0}\end{matrix} \right. & (1)\end{matrix}$

[0041] where α is the fiber attenuation and g is the Raman gaincoefficient between channel P_(p) and channelP_(s). For opticalperformance monitoring a sinusoidal dither (pilot tone) is added to eachchannel. If channel P_(p) is modulated by a single tone (sinusoidaldither m cos ωt) the equation for an undepleted channel can easily beobtained from the above equation as:

P _(p)(z)=P _(0p) e ^(−az)(1+m cos ωt)  (2)

[0042] in which P_(0p) is the mean launch power. Substituting (2) into(1) gives rise into the following SRS Crosstalk for channel P_(s) due tothe superimposed dither on channel P_(p). $\begin{matrix}{{P_{s}(z)} = {P_{s0}{^{{- \alpha}\quad L}\left\lbrack {\left( {1 + \frac{P_{p0}g}{\alpha}} \right) + {\frac{P_{p0}g}{\alpha}m\quad {\cos \left( {\omega \quad t} \right)}}} \right\rbrack}}} & (3)\end{matrix}$

[0043] Compared to the situation without SRS, channel P_(s) sees adepletion of the mean level and some cross-talk from the P_(p)modulation. Modulation depth of the cross-talk m′ is given by:$\begin{matrix}{m^{\prime} = {\frac{{gP}_{p0}P_{s0}}{\alpha}m}} & (4)\end{matrix}$

[0044] which is identical to the SRS impact on average power given bythe second $\frac{{gP}_{p0}P_{s0}}{\alpha},$

[0045] term in (3). Recalling that the inaccuracy in pilot tone powerlevel estimation comes from the fact that the mean level of the averagepower is changed due to SRS according to$\frac{{gP}_{p0}P_{s0}}{\alpha}$

[0046] whereas the dither amplitude as detected by the pilot tonemonitoring apparatus does not see any change. The difference is actuallythe SRS inaccuracy in pilot tone power level estimation technique. Forthe two channel case this error is equal to$\frac{{gP}_{p0}P_{s0}}{\alpha}$

[0047] but for a multi-channel system (DWDM system) this simple solutionis not valid. This is because the SRS energy transfer from the firstchannel to the second channel increases the power of the second channeland therefore will contribute to the SRS error between the secondchannel and the third channel. The same concept is also true for allother channels in the system. Therefore, we have to extend the aboveresults in a way to accommodate the presence of more than two channels.

[0048] In order to get a better estimate of the SRS energy transferwhile taking advantage of using the simple solution of equation (3) wecan implement an iterative approach which incrementally calculates theSRS energy transfer between each pair and subsequently calculates theimpact between other pairs based on this incremental change. Thefollowing pseudo-code shows this approach for a single fiber based onequation (3). for k=1:2 for i=1:Nch for j=1:NchX(i)=X(i)+0.5*G*(j−i)*SpanP(j)*SpanTxP0; end SpanP(i)=SpanP(i)−X(i); endend

[0049] In the above notation we have:

[0050] Nch=Number of channels

[0051] SpanTxP0=Launch power into the fiber (equal power per wavelengthis assumed)

[0052] SpanP(j)=output power at channel j

[0053] X(i)=SRS error in channel i

[0054] In this code the SRS contribution of each channel on all otherchannels is first calculated in the internal loop. Then the externalloop uses the modified power levels to calculate the total SRS error foreach channel. FIG. 5 displays the results of this approach for 40channels with 6.0o dBm average power per wavelength in an 80 km NDSFfiber.

[0055] Step 2: Accumulation of SRS error in a multi-pan system

[0056] Once the SRS error for each channel over one span is given we cancalculate the total SRS error in a multi-span system. To simplify thecalculations we assume all spans are identical with a series combinationof fiber, dual amplifier, DCM and booster amplifier. Furthermore, weassume a flat gain characteristic for dual amplifier and a flat outputpower distribution for the booster amplifier.

[0057] Now consider the first span. If P₀ denotes the input power intothe fiber per wavelength then dual input power which is located at theend of the first span fiber will be equal to P₀β₁+x₁ where β₁ is spanfiber attenuation and x₁ is the SRS error coming from step 1.

[0058] The power at the input of the following booster is given by:

[0059] (P₀β₁A₁+x₁A₁)β₂+x₂ in which A₁ is the dual gain, β₂ is DCMattenuation and x₂ is the SRS error in DCM. Recalling that the DCMmodule comprises an internal fiber which adds to the overall SRS error.Since we have assumed that the booster flattens the power to P₀ for allwavelengths the booster output power will be obtained as:$\begin{matrix}{\frac{P_{0}^{2}}{P_{0} + {x_{1}A_{1}A_{2}\beta_{2}} + {x_{2}A_{2}}} + \frac{\left( {{x_{1}A_{1}A_{2}\beta_{2}} + {x_{2}A_{2}}} \right)P_{0}}{P_{0} + {x_{1}A_{1}\beta_{2}A_{2}} + {x_{2}A_{2}}}} & (5)\end{matrix}$

[0060] in which A₂ denotes the booster gain.

[0061] To derive equation (5) we first multiply the booster input power(P₀β₁A₁+x₁A₁)β₂+x₂ by A₂. Then we find a multiplier to flatten the powerto P₀. If η₁ denotes this multiplier we should have

[(P ₀β₁ A ₁ +x ₁ A ₁)A ₂β₂ +x ₂ A ₂]η₁ =P ₀  (6)

[0062] Therefore, $\begin{matrix}{\eta_{1} = \frac{P_{0}}{\left\lbrack {{\left( {{P_{0}\beta_{1}A_{1}} + {x_{1}A_{1}}} \right)A_{2}\beta_{2}} + {x_{2}A_{2}}} \right\rbrack}} & (7)\end{matrix}$

[0063] The booster output power will now be given by: $\begin{matrix}{P_{{Booster}\quad 1} = {\left\lbrack {P_{0} + {x_{1}A_{1}A_{2}\beta_{2}} + {x_{2}A_{2}}} \right\rbrack \frac{P_{0}}{\left\lbrack {P_{0} + {x_{1}A_{1}A_{2}\beta_{2}} + {x_{2}A_{2}}} \right\rbrack}}} & (8)\end{matrix}$

[0064] In equation (8) we further assumed A₁A₂β₁β₂=1 meaning that thetotal loss in each span is equal to the total gain in that span.

[0065] Now we define:$R_{1} = {{\frac{P_{0}^{2}}{P_{0} + {x_{1}A_{1}A_{2}\beta_{2}} + {x_{2}A_{2}}}\quad E_{1}} = \frac{\left( {{x_{1}A_{1}A_{2}\beta_{2}} + {x_{2}A_{2}}} \right)P_{0}}{P_{0} + {x_{1}A_{1}\beta_{2}A_{2}} + {x_{2}A_{2}}}}$

[0066] as the true power (^(R)) and SRS error power (^(E)) (induced byother channels). $\begin{matrix}{R_{k} = {{\frac{R_{k - 1}P_{0}}{\left( {R_{k - 1} + E_{k - 1}} \right) + x_{1}^{\prime} + {x_{2}^{\prime}A_{2}\beta_{2}}}\quad E_{k}} = \frac{\left( {x_{1}^{\prime} + {x_{2}^{\prime}A_{2}\beta_{2}} + E_{k - 1}} \right)P_{0}}{\left( {R_{k - 1} + E_{k - 1}} \right) + x_{1}^{\prime} + {x_{2}^{~\prime}A_{2}\beta_{2}}}}} & (3)\end{matrix}$

[0067] For the second span we inject the booster 1 output power(equation 8) to the next set of span fiber, dual amplifier, DCM, andbooster amplifier. Along the same line of derivation it is easy to showthe output power of the second booster at the end of the second span isgiven by: $\begin{matrix}{P_{{Booster}\quad 2} = {\left\lbrack {P_{{Booster}\quad 1} + {x_{1}A_{1}A_{2}\beta_{2}} + {x_{2}A_{2}}} \right\rbrack \frac{P_{0}}{\left\lbrack {P_{{Booster}\quad 1} + {x_{1}A_{1}A_{2}\beta_{2}} + {x_{2}A_{2}}} \right\rbrack}}} & (9)\end{matrix}$

[0068] Substituting equation (8) into (9) yields: $\begin{matrix}{P_{{Booster}\quad 2} = {\frac{R_{1}P_{0}}{\left( {R_{1} + E_{1}} \right) + {x_{1}A_{1}\beta_{2}A_{2}} + {x_{2}A_{2}}} + \frac{P_{0}\left( {{x_{1}A_{1}A_{2}\beta_{2}} + {x_{2}A_{2}} + E_{1}} \right)}{\left( {R_{1} + E_{1}} \right) + {x_{1}A_{1}\beta_{2}A_{2}} + {x_{2}A_{2}}}}} & (10)\end{matrix}$

[0069] Continuing the same procedure will produce the followingiterative approach for the kth booster:

P _(Booster k) =R _(k) +E _(k)

[0070] where $\begin{matrix}{R_{k} = {{\frac{R_{k - 1}P_{0}}{\left( {R_{k - 1} + E_{k - 1}} \right) + x_{1}^{\prime} + {x_{2}^{\prime}A_{2}\beta_{2}}}\quad E_{k}} = \frac{\left( {x_{1}^{\prime} + {x_{2}^{\prime}A_{2}\beta_{2}} + E_{k - 1}} \right)P_{0}}{\left( {R_{k - 1} + E_{k - 1}} \right) + x_{1}^{\prime} + {x_{2}^{\prime}A_{2}\beta_{2}}}}} & (3)\end{matrix}$

[0071] and

x₁=β₁x₁′, x₂=β₂x₂′

[0072] In other words the power for each wavelength has two componentsthe actual power and the power induced in the channel by SRS. Note thatthe SRS induced errors in each span (x₁ and x₂) are provided by step 1whereas SRS error accumulation over multiple spans is calculated by step2. Using these two steps together enables us to estimate the total SRSerror for each channel for a DWDM multi-span system. FIG. 6 illustratesthe SRS error in pilot tone power estimation for 40 channels 6-span NDSFsystem with 6.0 dBm launch power per wavelength.

[0073] Although the present invention has been described in considerabledetail with reference to certain preferred versions thereof, otherversions are possible. Therefore, the spirit and scope of the appendedclaims should not be limited to the description of the preferredversions contained herein.

[0074] All the features disclosed in this specification (including anyaccompanying claims, abstract, and drawings) maybe replaced byalternative features serving the same, equivalent or similar purpose,unless expressly stated otherwise. Thus, unless expressly statedotherwise, each feature disclosed is one example only of a genericseries of equivalent or similar features.

What is claimed is:
 1. An iterative method for Stimulated RamanScattering (SRS) error estimation in an optical fiber network, thenetwork characterized in that it comprises the infrastructure requiredto measure the power levels of all optical channels using a pilot tonemonitoring technique, the method comprising the steps of: (i)determining the multi-channel SRS error value for each pairing of allpossible pairings of all the channels by inputting measured values ofchannel power levels into a two-channel equation substantially equal to:${P_{s}(z)} = {P_{s0}{^{{- \alpha}\quad L}\left\lbrack {\left( {1 + \frac{P_{p0}g}{\alpha}} \right) + {\frac{P_{p0}g}{\alpha}m\quad {\cos \left( {\omega \quad t} \right)}}} \right\rbrack}}$

where α is the fiber attenuation and g is the Raman gain coefficientbetween channel P_(p) and channel P_(s); and (ii) calculating the SRSerror in a single fiber span for all channels by extending thetwo-channel equation in an iterative form substantially equal to: fork=1:2 for i=1:Nch for j=1:Nch X(i)=X(i)+0.5*G*(j−i)*SpanP(j)*SpanTxP0;end SpanP(i)=SpanP(i)−X(i); end end

to estimate the SRS error in a single span. where Nch=Number of channelsSpanTxP0=Launch power into the fiber (equal power per wavelength isassumed) SpanP(j)=output power at channel j X(i)=SRS error in channel iG=g/α
 2. The method according to claim 1, further comprising determiningfor each channel over multiple fiber spans the amount of total SRS errorvalue at every span based on the calculated SRS error value of itsprevious span so as to determine the SRS error accumulated over amulti-span network, the method comprising an iterative form of thetwo-channel equation substantially equal to:$R_{k} = {{\frac{R_{k - 1}P_{0}}{\left( {R_{k - 1} + E_{k - 1}} \right) + x_{1}^{\prime} + {x_{2}^{\prime}A_{2}\beta_{2}}}\quad E_{k}} = \frac{\left( {x_{1}^{\prime} + {x_{2}^{\prime}A_{2}\beta_{2}} + E_{k - 1}} \right)P_{0}}{\left( {R_{k - 1} + E_{k - 1}} \right) + x_{1}^{\prime} + {x_{2}^{\prime}A_{2}\beta_{2}}}}$


3. A system for Stimulated Raman Scattering (SRS) error estimation in anoptical fiber network, the network characterized in that it comprisesthe infrastructure required to measure the power levels of all opticalchannels using a pilot tone monitoring technique, the system comprising:means for determining the multi-channel SRS error value for each pairingof all possible pairings of all the channels by inputting measuredvalues of channel power levels into a two-channel equation substantiallyequal to:${P_{s}(z)} = {P_{s0}{e^{{- \alpha}\quad L}\left\lbrack {\left( {1 + \frac{P_{p0}g}{\alpha}} \right) + {\frac{P_{p0}g}{\alpha}m\quad {\cos ({\omega t})}}} \right\rbrack}}$

where α is the fiber attenuation and g is the Raman gain coefficientbetween channel P_(p) and channel P_(s); and means for calculating theSRS error in a single fiber span for all channels by extending thetwo-channel equation in an iterative form substantially equal to: fork=1:2 for i=1:Nch for j=1:Nch X(i)=X(i)+0.5*G*(j−i)*SpanP(j)*SpanTxP0;end SpanP(i)=SpanP(i)−X(i); end end

to estimate the SRS error in a single span where Nch=Number of channelsSpanTxP0=Launch power into the fiber (equal power per wavelength isassumed) SpanP(j)=output power at channel j X(i)=SRS error in channel iG=g/α
 4. The system according to claim 3, further comprising a means fordetermining for each channel over multiple fiber spans the amount oftotal SRS error value at every span based on the calculated SRS errorvalue of its previous span so as to determine the SRS error accumulatedover a multi-span network, the system comprising an iterative form ofthe two-channel equation substantially equal to:
 5. A system forStimulated Raman Scattering (SRS) error estimation in an optical fibernetwork, the network characterized in that it comprises theinfrastructure required to measure the power levels of all opticalchannels using a pilot tone monitoring technique, the system comprising:a network component having embedded computer readable code comprising atwo-channel equation for determining a multi-channel SRS error value foreach pairing of all possible pairings of all the channels by inputtingthe measured channel power levels into the equation, the equationsubstantially equal to${P_{s}(z)} = {P_{s0}{e^{{- \alpha}\quad L}\left\lbrack {\left( {1 + \frac{P_{p0}g}{\alpha}} \right) + {\frac{P_{p0}g}{\alpha}m\quad {\cos ({\omega t})}}} \right\rbrack}}$

where α is the fiber attenuation and g is the Raman gain coefficientbetween channel P_(p) and channel P_(s); and a network component havingembedded computer readable code comprised of an iterative extended formof the two-channel equation substantially equal to: for k=1:2 fori=1:Nch for j=1:Nch X(i)=X(i)+0.5*G*(j−i)*SpanP(j)*SpanTxP0; endSpanP(i)=SpanP(i)−X(i); end end

to estimate the SRS error in a single span where Nch=Number of channelsSpanTxP0=Launch power into the fiber (equal power per wavelength isassumed) SpanP(j)=output power at channel j X(i)=SRS error in channel iG=g/α
 6. The approach according to claim 5, further comprising a networkcomponent having embedded computer readable code capable of determiningfor each channel over multiple fiber spans the amount of total SRS errorvalue at every span based on the calculated SRS error value of itsprevious span so as to determine the SRS error accumulated over amulti-span network, the code comprised of an iterative form of thetwo-channel equation substantially equal to:$R = \frac{R_{k - 1}P_{0}}{\left( {R_{k - 1} + E_{k - 1}} \right) + x_{1}^{\prime} + {x_{2}^{\prime}A_{2}\beta_{2}}}$$E_{k} = \frac{\left( {x_{1}^{\prime} + {x_{2}^{\prime}A_{2}\beta_{2}} + E_{k - 1}} \right)P_{0}}{\left( {R_{k - 1} + E_{k - 1}} \right) + x_{1}^{\prime} + {x_{2}^{\prime}A_{2}\beta_{2}}}$